Cd2 ion is known to uncouple mitochondrial oxidative phosphorylation and stimulate the ATPase at micromolar levels. The effect is reversed by 2,3-dimercaptopropanol but not by monothiols. We propose to label the cadmium binding component, probably a protein, with radioactive Cd2 ion, isolate it and characterize it. Its function will be determined after removal of the Cd2 ion by washing with excess, 2,3-dimercaptopropanol. The implication of the Cd2 ion binding to its toxicity will be explored.